Single atom stores quantum information

May 3, 2011

A single atom (purple sphere) is trapped at the center of an optical cavity (formed by the coned mirrors in blue) using a standing-wave dipole trap (green). An impinging weak coherent probe pulse (red) is converted into an atomic spin excitation. (Credit: Max-Planck-Institut für Quantenoptik/Nature)

Researchers at the Max Planck Institute of Quantum Optics in Garching, Germany have stored quantum information in a single atom.

The researchers wrote the quantum state of single photons into a rubidium atom and read it out again after a discrete storage time.

They say this technique can be used in principle to design powerful quantum computers and to network them with each other across large distances.

The photons carried the quantum information in the form of polarization. This can be left-handed (the direction of rotation of the electric field is anti-clockwise) or right-handed (clock-wise).

The quantum state of the photon can also contain both polarizations simultaneously as a superposition state.

In its interaction with the photon, the rubidium atom is usually excited and then loses the excitation again by means of the probabilistic emission of a further photon.

The researchers wanted instead to bring the rubidium atom into a definite, stable quantum state. They achieved this with the aid of a control laser beam that was directed onto the rubidium atom at the same time as it interacted with the photon.

The storage process maps the photonic polarization qubit onto a long-lived superposition of the ground states of the atom. The purple spheres and the red cloud signify the atomic population and a coherent superposition, respectively. (Credit: Max-Planck-Institut für Quantenoptik/Nature)

Irradiating the rubidium atom with the control laser caused it to re-emit the photon. In most cases, the quantum information in the read-out photon agreed with the information originally stored. The fidelity was more than 90 percent.

This is significantly higher than the 67 percent fidelity that can be achieved with classical methods not based on quantum effects. The method is therefore a true quantum memory, the physicists said.

However, the physicists said that the efficiency, the measure of how many of the irradiated photons are stored and then read out again, was just under 10 percent.

They also measured the storage time (the time the quantum information in the rubidium can be retained) as approximately 180 microseconds. While comparable with the storage times of all previous quantum memories based on groups of atoms, longer storage time will be necessary for the method to be used in a quantum computer or a quantum network, the physicists said.

Ref: Holger P. Specht, Christian Nölleke, Andreas Reiserer, Manuel Uphoff, Eden Figueroa, Stephan Ritter, Gerhard Rempe, A single-atom quantum memory, Nature, 2011; DOI: 10.1038/nature09997